1. Field of the Invention
The present invention relates to wireless communication and, more particularly, to a user scheduling method in a multi-antenna system.
2. Description of the Related Art
Due to generalization of information communication services and the advent of various multimedia services and quality services, demand for communication services is increasing. To meet the demand, diverse wireless communication technologies are under study in multiple sectors.
A multi-input multi-output (MIMO) technique receives attention as a scheme for enhancing link reliability and preventing interference as well as having high frequency efficiency. The MIMO technique can increase a diversity gain by allowing signals transmitted via multiple transmit antennas to have independence.
The MIMO scheme obtains gain through diverse precoding such as beamforming or space time coding. The space time coding may be divided, for example, into a space time block coding (STBC) and a space time trellis coding (STTC). The STBC obtains diversity by appropriately adjusting the same stream for each transmit antenna during one transmission period. With the STBC, system diversity is increased linearly in proportion to the number of antennas, but the throughput does not increase. The STTC is a scheme for determining a signal to be transmitted next based on Trellis coding, which has a more complexity than the STBC. In the MIMO system, a diversity gain and a spatial multiplexing gain appear differently according to a channel environment. In a room with a large angle spread of a reception signal or in a sufficient scattering environment, the diversity gain can be maximized through space time coding, while in an environment in which the angle spread of a reception signal is small and a line-of-sight (LOS) is guaranteed, throughput can be maximized through the space multiplexing scheme. In other words, because demands of users to be met vary depending on the situations of users, those demands cannot be satisfied 100% with solely the STC scheme or solely the SDM scheme.
Meanwhile, besides the MIMO scheme, an adaptive modulation and coding (AMC) scheme is frequently used to enhance performance of the system. The AMC scheme is a technique of increasing link performance by adjusting a transmission power level, a modulation order and/or a code rate in a transmitter by using channel state information (CSI). Throughput is allowed to be increased when a channel state is good, and allowed to be lowered in the occurrence of channel degradation, to thereby support effective transmission, and resultantly, an average throughput can be increased.
In the MIMO system employing various preprocessing (preceding) and AMC scheme, user scheduling does matter. With available radio resources limited, which users should be selected from among those in a cell and which radio resources should be allocated to the selected users are problematic to obtain maximum performance, for which, thus, user scheduling is considered.